Boy Howdy there !
When racin' stops being fun, your passion and performance as a driver suffer. Then the "Ef it" attitude worms it's way in to every facet of your operation.
The absolute lowest class a track has is ALWAYS going to evolve into a faster(more expensive) class. That's why you'll see tracks introduce new 'entry level' classes every so often. Purty soon THEY are so competitive and expensive they have to start ANOTHER 'entry level' class, and on and on.
Another big factor for the domestic 'bomber/hobby stock classes evolving into Street Stock, is the burning rate of domestic 'stock' cars to build into race cars. They're all being snatched up for muscle car restos/build ups,( OR THEY'VE BEEN CRUSHED for $$$$$$) etc.,so there's nothing left to use as circle burners. That's why they've started so many 4 cylinder classes, or recycled old tube framed super stocks into street stocks.